Proper functioning of motor nerves is critical to survival and even minor impairments can have a negative impact on the socioeconomic capacity of an individual's life. Even non-life threatening conditions such as hemifacial spasm and other focal dystonias can progress to a point where the quality of life is severely impaired.
Many types of nerves are very complex, for example the facial nerve is formed from the motor neurons that innervate the facial muscles and from the nervous intermedius of Wrisberg which contains preganglionic parasympathetic fibers. As such it is a mixed nerve with special visceral efferent, general visceral efferent, special visceral afferent, and general somatic afferent functions.
Improper functioning of nerves such as this can result from many causes including infection, tumor growth, and trauma. The pathophysiological mechanisms underling hemifacial spasm, for example, include the ectopic generation of discharges which can reach up to 150 impulses per second with irregular burst repetitions, ephaptic transmission and the lateral spread of excitation between facial axons. After continuous antidromic bombardment of inputs, a hyperexcitability of facial motoneurons can occur, the initial cause in most cases being a zone of demyelination at the root exit zone of the facial nerve.
Treatment and rehabilitation of many nerve diseases can be very difficult depending on the type and stage of development. In many cases, however, the etiology is not known, such as in benign essential blepharospasm, meige disease, and other related dystonias. For hemifacial spasm, treatment includes Botulinum toxin injections, facial nerve denervation, and surgical decompression of the offending vessel or vessels.
Disruption of aberrant neuronal activity patterns through stimulation is a useful approach to treating movement disorders, for example, electrical deep-brain stimulation has been found to be a helpful complement to dopamine replacement therapy for treating motor symptoms in Parkinson's disease. The low-frequency synchronous cortiostriatal oscillations can be controlled, but at the cost of a highly invasive surgical procedure which implants electrodes in such areas as the basal ganglia. Here the metal electrodes create an electrical field that affects the nearby cells. In order for the electrical pulses to activate the neuronal elements, it must drive ions across the cell membrane, and the lines of electrical force from the cathode to the anode must therefore be perpendicular to the membrane. Mutipolar electrodes are used to shape the electric field to more closely conform to the complexity of the tissue being treated and allows for more flexibility in spread and intensity. A useful overview of this can be found in “Deep Brain Stimulation Programming” by Montgomery, Feb. 20, 2006.
A more recent approach looks at implanting light-activated chloride pumps into the primary excitatory neuron in the subthalamic nucleus. (see page 1555, Science, Vol 323 Mar. 20, 2009). Stimulating peripheral nerves to affect oscillatory neuronal activity in the brain would be a less invasive approach, and is of considerable current interest.
Transcranial magnetic stimulation has also been used over the years to affect the behavior of nervous tissue and has been applied to peripheral nerves with some efficacy. It operates on the principal of induction which produces a magnetic field oriented orthogonally to the plane of the coil. The current induced in the structures of the brain activates nearby nerve cells in unpredictable ways, even with the advent of new coil types such as the Hesed Coil which allows for deeper penetration without inducing a much greater stimulation of superficial cortical regions.
While the above techniques are useful in stimulating neuronal tissue, in many cases of disease, and especially in dystonias, what is desired is the quieting, not the additional stimulation, of harmful signals which cause uncontrolled motor responses. Because these harmful signals typically form a stochastic process, and are embedded within a stream of otherwise healthful signals, then what is desired is a method to control them without harming the nerve, its components, or the surrounding tissue. Also, because in may of the dystonias the exant cause of continuation of spasm is a harmful feedback loop, the need for continuous control is lessened, as intervening in a selected few time intervals can be sufficient to uncouple the feedback from the source.